1. Field of the Invention
The present invention relates to a pigment dispersant. More specifically, it relates to a pigment composition, a pigment dispersion and a printing ink which contain the pigment dispersant.
2. Description of the Related Art
In a printing ink, a paint coating or the like, a pigment is dispersed in a fine state, whereby printed matters or coated articles have properties such as a clear color tone and gloss. Moreover, when the pigment is dispersed in a stable state, flowability of the printing ink or the paint coating is improved, which leads to the improvement of printability. In addition, labor and energy can be remarkably reduced during the manufacture of the printing ink or the paint coating. Furthermore, when a dispersion is in a stable state, it is generally excellent in storage stability.
The pigment used in the printing ink or the paint coating often has a fine particle diameter in order to realize a clearer color tone. However, when the particles of the pigment are fine, aggregation between the pigment particles becomes strong, and hence it is often difficult to obtain such properties as mentioned above.
In order to solve such a problem, it has been known that a pigment dispersant is used to enhance affinity between the pigment and a vehicle, thereby stabilizing the dispersion. Hitherto, various kinds of pigment dispersants have been disclosed.
For example, as disclosed in Japanese Patent Publication No. 2466/1966 and U.S. Pat. No. 2,855,403, it has been known to use compounds obtained by introducing a side chain substituent selected from a sulfonic acid group, a sulfonamide group, an aminomethyl group, a phthalimidomethyl group and the like into an organic pigments as a main structure. These compounds have large effects of a non-aggregating property, crystal stability and the like, but they have characteristics close to dyes. Accordingly, they are interior to pigments in durability in some cases. Therefore, when a pigment composition for inks and paint coatings contains these compounds, hues of the compounds fade with time, thereby, hues of coating films change in some cases. Moreover, after coated films are formed, these compounds come to migrate to the surfaces of the coated films by a high-temperature treatment such as boiling or baking or by pressure attachment, thereby, a migration of color occur in some cases.
Furthermore, in Japanese Patent Application Laid-Open Nos. 127749/1996 and 199796/1999, there is disclosed, for solving these problems, basic compounds each containing a triazine ring which does not have a coloration inherent in the pigments and which has versatility for the various pigments. These compounds show an effect when used in combination with a binder having an acidic functional group such as an acrylic resin or an alkyd resin, but they have not exhibited the sufficient effect in the case that a naphthol pigment and a condensed azo pigment are contained in the pigment. Moreover, in the case that the compounds are used in combination with a binder having a basic functional group such as a polyurethane resin or a polyamide resin, they have hardly exhibited the effect. In particular, in the case that there is used a highly basic polyurethane resin containing a urea bond which is used for gravure inks and the like, any effect has not been exhibited.
Namely, it is desired to develop a pigment dispersant which is excellent in flowability and dispersion stability in the case that it is used in a printing ink or a paint coating and which is excellent in gloss in the case of the formation of dry films. In particular, a pigment dispersant is desired which is effective for naphthol pigments and condensed azo pigments.